This invention relates to the field of developing focus supply voltages for cathode ray tubes in televisions and the like.
A high voltage power supply, for example in a television receiver, generates a high voltage from the high voltage winding in a flyback transformer, sometimes constructed as an integrated high voltage transformer (IHVT). The voltage is boosted by a series of windings and diodes. The high voltage (HV), sometimes referred to as EHT (extra high tension) may be approximately 30 KV or higher, depending on the size of the cathode ray tube. Some receivers with larger tubes utilize high voltage boosters having a second transformer, a second horizontal output stage and a second high voltage winding. The horizontal focus voltage supply in a television receiver may be in the range of approximately one fifth (1/5) to one third (1/3) of the EHT. Focus voltage and screen voltage can therefore be derived from the EHT.
A focus screen assembly is usually provided for generating the focus and screen voltages. The focus screen assembly, which is energized by the high voltage winding of the IHVT, includes a network of fixed resistors, variable resistors and capacitors. The assembly will generate horizontal focus voltage, also referred to as Focus 1 voltage, screen voltage and vertical focus voltage, also referred to as Focus 2 voltage. The component resistors of the assembly are deposited on a ceramic substrate and the assembly is fully enclosed and insulated. Means for adjusting the variable resistors to set the focus and screen voltage levels are accessible from outside the case. The assembly is coupled to the high voltage output of the IHVT, also referred to as the flyback transformer. The assembly utilizes a resistor chain to supply the focus and screen voltages at levels below the high voltage level.
There are two approaches known for energizing a focus screen assembly, and appropriate focus screen assemblies are manufactured for each of these approaches. One approach is generally referred to as the resistor divider network approach. The other approach is generally referred to as the peak detected approach, although a resistor chain is also utilized in the focus screen assembly. The approaches differ in the point of the high voltage winding from which energy is coupled to the focus screen assembly.
The resistor divider network approach, such as shown in FIG. 1, energizes the focus screen assembly with the full value of the EHT, as supplied to the cathode ray tube. This approach tends to provide a rather large source impedance for the focus supply, for example approximately 170 MOhm, which can become a problem for focus leakage currents. Focus leakage currents can be caused by imperfections, impurities and gas particles in the tube. It will be appreciated that a leakage current of only 70 .mu.amp, for example, will cause a large voltage drop against an impedance of 170 MOhm. One might try to reduce the source impedance of the resistive divider, but two problems in particular are encountered. Firstly, reducing the source impedance results in a higher current density, which requires a larger ceramic substrate area to handle the additional heat dissipation. This adds to the expense of the assembly. Secondly, it is costly in general to generate the high voltage in the first place. Energy loss through such heat dissipation is inefficient and costly, in and of itself. On the other hand, the resistive divider desirably provides a small bias current for the high voltage supply even when the cathode ray tube is not conducting. High voltage supply changes at low beam current levels are therefore reduced. As a consequence, raster size changes resulting from changes in beam current are minimized.
The peak detected approach, such as shown in FIG. 2, energizes the focus screen assembly from a tap of the high voltage winding of the IHVT. The voltage at the tap should be greater than the required focus voltage, so that an adjustment range is available. A one third (1/3) tap, that is, a voltage level equal to approximately one-third (1/3) of EHT, can be appropriate. This approach has the advantage of providing a lower source impedance, but disadvantageously, does not provide much preloading, in the form of a bias current for the high voltage supply. As a consequence, raster size changes resulting form changes in beam current are more likely.
The problem, then, is to provide a supply voltage for the focus screen assembly from a low impedance source, which minimizes voltage loss and dissipation, and at the same time, provide an adequate bias voltage for the high voltage supply, even when the cathode ray tube is not conducting.